1. Field of the Invention
The present invention relates to an apparatus for disinfecting water by the direct introduction of gaseous ozone into the water. More particularly, the present invention relates to a water treatment apparatus in which gaseous ozone is introduced into a vertically disposed water conduit by a plurality of porous diffuser elements positioned within the conduit.
2. Description of the Related Art
The notion of treating water with gaseous ozone for disinfection is well known. Various types of apparatus and methods have been devised to introduce and dissolve gaseous ozone into water for treatment of the water by providing retention chambers in which the ozone is introduced through mechanical mixers, injectors, porous diffusers, and packed columns. The primary purpose of each of those approaches is to transfer sufficient gaseous ozone to the water to be treated, in order to provide the required contact time of the ozone with the water so that the disinfection treatment is as thorough as possible.
Although there are several known approaches to accomplish the disinfection operation, even the more efficient of those processes, in terms of the efficiency of exposure of the water to ozone, include certain drawbacks. For example, although packed column technology provides transfer efficiencies of from about 80% to about 95%, there is little operational experience with such approaches, and, more significantly, there is a strong potential for high head losses as the result of build-up of scale on the column medium. Mechanical mixing, on the other hand, although capable of providing ozone transfer efficiencies in the range of from about 90% to about 98%, involves short ozone-water contact times that might require retreatment for effective disinfection. Additionally, mechanical mixing also involves high operating cost because of the energy usage and the requirement for maintenance of the mechanical equipment.
Fine bubble diffuser contactors, another approach for effecting disinfection of water by the introduction of ozone, are quite widely used and provide ozone transfer efficiencies in the range of about 90% to about 95%. They operate effectively at low to medium ozone concentrations (&lt;6% by wt.) when there is a sufficient volumetric ozone flow rate delivered to the ozone contactor to effectively distribute the ozone to the diffusers and provide adequate ozone-water mixing for homogenous ozone dissolution. However, at higher ozone concentrations (8 to 12% by wt.) and lower gas flow rates, which often result when oxygen feed gas systems and advanced technology ozone generators are employed, there is an increased risk of gas bubble channeling, with resultant inadequate contact between the ozone gas bubbles and the water being treated, which could cause incomplete disinfection of the water.
To overcome inefficient mixing conditions in fine bubble diffuser systems at higher ozone concentrations, the present inventor developed an improved ozone dissolution system, combining a sidestream venturi injection system with a specially designed downflow tube to convey the ozone-water mixture into an ozone contactor. The downflow tube is installed inside one or more diffusion chambers of the ozone contactor to increase the time of contact of the ozone with the water. Turbulent mixing conditions are provided by the sidestream venturi injection system, even at low plant flows or low ozone doses. Pressurized sidestream flows discharge into the downflow tube through opposing dual venturi injectors, which also pull ozone gas onto the sidestream flows under negative pressure. In addition, the vertically oriented downflow tube maximizes bubble hold-up, thereby increasing bubble contact time with the water and improving ozone mass transfer rates. An example of such a system is disclosed in the present inventor's U.S. Pat. No. 5,273,664, which issued on Dec. 28, 1993, and is entitled, "Apparatus and Method for Disinfection of Water by Ozone Injection." That patent discloses the combination of sidestream injection and injection within the downflow tube to introduce ozone into the water.
Although the sidestream injection and downflow tube apparatus and method disclosed in the above-mentioned patent provides distinctly improved ozone transfer efficiency, it does so at a higher capital cost and complexity, as a consequence of additional piping, booster pumps, and instrumentation and control requirements. Additionally, very high mixing gradients (600 to 1,000 sec.sup.-1) are created in the downflow tube by the combined energy input of the ozone gas and the pressurized side stream flows, which can easily shear floc particles in settled water if the ozone contactor is installed downstream of coagulation and sedimentation water treatment processes. Based on those factors, that approach appears to be best suited for preoxidation (preozone) applications at the head of the treatment plant. At that location in the treatment process, raw water is typically pumped to the preozone contactor so higher head losses across the downflow tube will not impact the plant hydraulic grade line to any extent. Moreover, floc shearing is not a concern, since the ozone is applied upstream of any coagulation.
There have been recent advances in ozone generation technology by several equipment suppliers, and it is now possible to generate ozone at concentrations as high as 15% by weight. However, there still exists a need to develop a cost-effective ozone contacting device that is fully compatible with those equipment advances, and that can be used effectively for primary disinfection of drinking water. In particular, the inactivation of Cryptosporidium requires higher ozone concentrations and longer contact times than for the inactivation of other microorganisms of interest (e.g., Giardia and viruses). Consequently, careful attention to ozone mass transfer and homogeneous, two-phase mixing considerations in the ozone contactor are of critical importance.
It is an object of the present invention to provide a simplified and efficient apparatus for introducing low volumetric flows and high concentrations of ozone into water and for maintaining longer and more turbulent ozone-water contact for providing more complete disinfection of water.